The luminous intensity (brightness) of a lamp made up of multiple LEDs is the result of the total luminous flux emitted from all the LEDs. To perform smooth and continuous brightness control of the lamp, the luminous flux emitted by the individual LED has to be adjusted. The adjustment of luminous flux of each LED can be achieved either by changing the amplitude level or the duty-cycle pulse, or by concurrently changing both the amplitude level and the duty-cycle pulse of the currents flowing through the LED.
Lamps that have adjustable CCT of wide ranges are highly valued products in the electric lighting market. Lamps with such a feature typically allow the continuous change of the CCT from a low value, e.g. 2000K (warm white) to a high value, e.g. 5000K (cold white). To achieve this, the lamp must comprise light sources with at least two distinct CCT values. In the case of LED lamps, an array of LEDs with low CCT (e.g. 2000K) and an array of LEDs with high CCT (e.g. 5000K) may be adopted in the product. If light of 2000K is required, only LEDs with CCT of 2000K are turned on. If light of 5000K is required, only LEDs with CCT of 5000K are turned on. For light of CCT between 2000K and 5000K, both arrays of LEDs are turned on and driven such that the overall combined light emitted from the lamp is of the required CCT value.
For example, in the method proposed by Miao (U.S. Pat. No. 8,159,125 B2 April 2012) [2], light from the two arrays of LEDs are mixed to give a desired CCT by controlling the proportion of the emitted light of each respective array. For this approach, the control of the CCT of the overall light is based on the formula:CCTlight=CCTlow*W+CCThigh*(1−W),  (Equation 1)
where CCTlow is the CCT value of the LEDs with the lower CCT, CCThigh is the CCT value of the LEDs with the higher CCT, and W is the weightage factor that allows the adjustment of the CCT. Here, W is bounded between 0 and 1 such that 0<W<1.
In the method proposed by Jonsson (20120146505) [3], the two arrays of LEDs are placed in anti-parallel manner such the anode of one LED array is connected to the cathode of the other LED array and vice versa. Current flowing in one direction turns on the first LED array and current flowing in the opposite direction turns on the second LED array. The driving actions are alternatively repeated. A controller manages the control of the CCT by adjusting the duty cycle D of an alternating current flowing through the two LEDs to control the color temperature and/or the brightness of the lighting apparatus. The control of the CCT of the light using such an approach can be mathematically expressed as:CCTlight=CCTlow*D+CCThigh*(1−D),  (Equation 2)
where 0<D<1.
In practice, however, the correlated color temperature (CCT) of the emitted flux of the LED changes with many factors, including the junction temperature of the respective LED and the amplitude of the current flowing through the LED. Therefore, with the prior approaches for adjusting brightness, there will always be an undesired change on the CCT of the luminous flux of the LEDs in the process of adjusting brightness. The change in CCT during the brightness adjustment process may or may not be significant. A ±200 K deviation in CCT within the desired CCT value is often cited as an acceptable error in electric lamps. Table 1 gives the requirement set in the ANSI Standard C78.377 [1].
TABLE 1Nominal CCT Categories (extracted from [40])Nominal CCTTarget CCT and Tolerance (K)2700 K2725 ± 1453000 K3045 ± 1753500 K3465 ± 2454000 K3985 ± 2754500 K4503 ± 2435000 K5028 ± 2835700 K5665 ± 3556500 K6530 ± 510
As described above, the CCT control approaches by Miao (U.S. Pat. No. 8,159,125 B2 April 2012) [2] and Jonsson (20120146505) [3] are based on simple linear relationships between the CCT of the component arrays of LEDs. As a result, the overall CCT control achievable with these approaches (hereinafter termed “linear approaches”) is highly inaccurate. The errors introduced by such prior approaches are significant especially if wide-range dimming and CCT control are required. FIG. 1 is a graph that shows the errors associated with prior approaches to controlling overall CCT of an LED lighting system comprising LEDs having two different CCTs. It is evident from FIG. 1 that there is deviation between the desired Ca control using linear approaches and the actual experimental CCT of the LED lighting system. The error is particularly significant at the higher desired. CCT level of 4000 K.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.